So, I'm watching some YouTube videos of A320 series takeoffs. Airbus are renowned for the 'double flash' strobe effect, but on a lot of the videos it seems as there is only one distinct flash. I know that there are actually two high intensity strobes per wing, but are they actually sequenced to *double flash*?

Reading the AMM, I can see that they are timed at 60 flashes per minute, +/- 10%. Is it just this 10% tolerance that provides the unique effect, and that theoretically they should flash at the same instant?

The wingtip strobes are double flash and the tail strobe is a single flash. Strobe lights are very hard to see in videos because the flash is so quick. Sometimes the flash is near the frequency if the shutter speed. You'll notice many telivision shows and movies use police cars, ambulances, etc with flashing halogen or slow flashing LED lights to combat this effect. In this video you can see, the strobe lights on the front of the ambulance, above the cab, and above the front wheel look like they are off except for random bursts on the camera, while the slow flashing lights on the side are easy to spot.

Quoting Shamrock137 (Reply 1):Sometimes the flash is near the frequency if the shutter speed.

I kind of guessed this may be the case, but with the tolerance then over time if one strobe unit is -10 per minute and the other is +5, then they will eventually come to a point where they flash simultaneously, won't they?

In the AMM, it says that they should provide +/- 120 flashes per minute, so they must be sequenced as you say - but I don't think it is a dispatch requirement for them to flash at this rate.

Quoting tom355uk (Reply 2):but I don't think it is a dispatch requirement for them to flash at this rate.

You are very right, certainly not a dispatch requirement. It is even possible to go with only one of the two double flash strobes operating. As far as the strobes blinking at the same time, while I'm sure it could happen, its probably very rare. Modern strobe modules usually do not go out of phase as they get older. All in all I think it would be a very rare event, and most likely what you are seeing is either from camera effects, or a burnt out strobe bulb.

Quoting tom355uk (Reply 2):In the AMM, it says that they should provide +/- 120 flashes per minute, so they must be sequenced as you say - but I don't think it is a dispatch requirement for them to flash at this rate.

Nope in fact they are deferrable and can be inop as long as the red anti collision lights are working. On the Airbus the pilots have the choice of auto which the strobes will come on after takeoff and go off after landing or they can select them on at any time or off.